1. Field of the Invention
The present invention relates generally to valves and, more particularly, to a valve which is provided with the capability of determining the operative state and condition of the valve without changing its actuation state.
2. Description of the Prior Art
Many different types of valves are known to those skilled in the art. One particular type of known valve is a spool valve in which a spool member is moved axially in response to pressure provided by a pilot valve and the axial movement of the spool member changes the interconnection between various ports formed in a valve body.
In certain applications, valves of the type described above are used in applications where they are intended to remain in a constant state of actuation for very long periods of time. During those extended time periods, the spool member of the valve does not move and the fluid interconnection between valve ports does not change. This continual sedentary state can lead to possible dangerous conditions when the valve is of the fail safe type and the sedentary state is in the actuated position. Under these circumstances, a failure in the valve's ability to return safely to its rest position can lead to disastrous consequences. When a valve, or any other mechanical device, is left in one condition for extended periods of time, the components which are intended to be moveable relative to each other can seize together and become frozen in their actuated position.
As an example of the above problem condition, a valve can comprise a solenoid operated pilot valve which controls the position of a spool member. In the actuated position, an electric current is continually provided to a solenoid coil of the pilot valve which causes a plunger of the pilot valve to move to and remain in an actuation position. This actuation position connects a motor portion of the spool member in fluid communication with a pressure source. This continuous pressure causes the spool member to remain in its actuation position. Both the spool member of the main valve and the plunger of the pilot valve are typically provided with spring return mechanisms that return them to their unactuated positions when the pressure is removed from the spool member because the current is removed from the solenoid coil. If a power failure occurs and the solenoid coil is deprived of electrical current, the valve is designed so that the lack of power will cause the springs in the pilot valve and main valve to urge the plunger and spool member back to their unactuated positions. Applied in this way, a power failure will result in the valve returning to its fail safe condition. Any equipment, such as pneumatic actuators, which are connected to the valve would typically be arranged in such a way that the return of the valve to its fail safe condition would result in all associated equipment being placed in a safe condition. The fail safe techniques described above depend completely on the return of the spool member to its unactuated position in response to the force provided by an internal spring. However, as is well known to those skilled in the art, when a valve is maintained in a constant position for an extremely long period of time, it can possibly remain permanently frozen in that position even after the actuation force provided by the solenoid pilot valve is removed. This seizure of the spool member within the valve housing can result from several causes. First, any two materials that are placed in intimate contact with each other for long periods of time can adhere to each other. This can result from molecular exchange between the materials or the build up of corrosion at the interface between them. In addition, the elastomeric seals which are typically used in valves can adhere to the surface of the spool member with which they are in contact. Regardless of the numerous reasons why the spool member can stick to associated components and remain in its actuated position after the pilot valve solenoid has been de-energized, it should be understood that a failure of this type can be catastrophic. It should also be understood that failures of this type are insidious because they do not become evident until after they occur. A valve which has been in an actuated position for an extended period of time can appear to be in perfect operating condition as long as it is actuated, but it can also be in a condition wherein the spool member is actually seized in its actuated position and will not respond by returning to its unactuated position if a power failure occurs or if the solenoid pilot valve is manually deactuated. These types of latent failures can also be very expensive to detect by manually turning the valve off for a moment and then back on. The process which is controlled by the valve may be one that does not lend itself to manual interruption in this manner.
Another expensive solution to this problem is to provide dual valve configurations in which the failure of one valve will not defeat the fail safe arrangement. However, this technique also has a serious drawback in that a failure of one of two redundant valves will not be readily evident as long as the other one of the two redundant valves is operating properly. This technique therefore merely delays the catastrophic failure.
In view of the above discussion of the problems related to valves which remain in an actuated position for long periods of time, it can be seen that it would be significantly beneficial if a means for testing the operability of an actuated valve is provided wherein the operating state of the valve need not be affected.